Malfunctioning of high voltage apparatus, such as transformers and on load tap changer (OLTC), can be both dangerous and expensive. Failure of the apparatus may cause an explosion and damage other equipment and humans.
High voltage apparatus, i.e. apparatus having a system voltage of more than 6 kV, often uses a fluid providing cooling and/or electrical insulation of the apparatus. In many apparatus such as transformers, electrical coils are encapsulated in a housing, which is filled with a fluid. The fluid is used for dissipation of heat while at the same time have an insulating functionality. Examples of such fluids may be oils, such as vegetable oil, mineral oil or synthetic or natural esters, or a gas, such as SF6.
In order to monitor normal behavior or to protect and detect malfunction of the apparatus, physical properties of the fluid, such as moisture content, hydrogen content, and pressure, are measured and supervised on-line, i.e. while the apparatus is in operation. Normal function or malfunction of the apparatus is monitored or detected based on the measured properties. The physical properties of the fluid may indicate that a fault or explosion has taken place. Detector equipment, such as sensors and other measuring utilities are positioned on the apparatus. To prevent the occurrences of incorrect reading or false trip of the apparatus or the system inspection and maintenance or calibration are regularly performed on the apparatus using detector equipment to ensure that the apparatus works correctly. For safety reasons and considering the high electrical potential in the area, the apparatus must be taken out of operation before such work can be performed. To be able to carry out inspection and maintenance or calibration of the encapsulated parts of the high voltage apparatus, the apparatus also has to be taken out of operation. However, to take a high voltage apparatus, such as a transformer, out of operation is time-consuming and costly, and therefore should be avoided. For modern transformers and tap changers, controls of the encapsulated parts of the apparatus are typically needed about every 10 to 15 years. However, control of the detector equipment is often needed more frequently, for example every 2 to 5 years.
Tap changers are used for controlling the output voltage of a transformer by providing the possibility of switching in or switching out additional turns in a transformer winding. A tap changer comprises a set of fixed contacts, which are connectable to a number of taps of a regulating winding of a transformer, where the taps are located at different positions in the regulating winding. A tap changer further comprises at least one moveable contact, which is connected to a current collector at one end, and connectable to one of the fixed contacts at the other end. By switching in or out the different taps, the effective number of turns of the transformer can be increased or decreased, thus regulating the output voltage of the transformer.
The regulating winding is typically not part of the tap changer, but forms part of the transformer. For example, the regulating winding is typically immersed in a transformer fluid, e.g. oil or SF6, while the tap changer is insulated with a separate insulation fluid. By separating the insulation fluids, the risk that the transformer is contaminated by dirt etc. from the tap changer is reduced. Hence, some type of barrier board is typically placed between the regulating winding and at least part of the tap changer, the barrier board preventing the two insulation fluid volumes from mixing. High voltage transformer tanks are very large. In order to reduce the volume of the transformer tank, the tap changer is sometimes mounted on the outside of the transformer tank, in a separate tap changer tank, which is mechanically attached to the transformer tank. But more commonly, a tap changer head is positioned on top of the transformer tank and the rest of the tap changer is placed inside the transformer tank.
Detector equipment for a tap changer may include a pressure relay designed to react on a pressure wave generated from a severe fault within the oil volume of the tap changer. The detector equipment for a tap changer may further include a pressure relief device, an oil flow relay, gas sensor, and/or a moisture sensor. The detector equipment is normally placed directly on or above the tap changer that is assembled on top of the transformer tank. Some of these transformer tanks have a height of several meters from the ground level. In order to reach the detector equipment, a technician needs to reach the top of the apparatus.
US2012/024187 discloses a sensor assembly for a sensor measuring the hydrogen concentration in an insulating fluid in an electric power generation, transmission and distribution apparatus. The sensor is mounted in a tube having a valve for blocking the oil flow while inspecting the sensor. The tube is extending into the apparatus to provide access to the interior of the apparatus and accordingly to the fluid. The tube forms an enclosure for the sensor. During inspection of the sensor, the sensor can be removed from the tube from the outside of the apparatus. The valve is used to block the oil flow after the sensor has been removed. However, it is still necessary to turn off the apparatus during inspection and maintenance of the sensor.
US2011/0093216 discloses a system comprising a container including an expansion tank arranged above a transformer and a pipe containing insulation fluid arranged between the container and the transformer. The container is partially filled with insulation fluid. A temperature sensor and a relative humidity sensor are positioned in the container. The temperature sensor is positioned in the insulation fluid and measures the temperature of the fluid in the container, and the relative humidity sensor is positioned in a gas above the insulation fluid and measures the humidity of the fluid. The temperature is measured in several positions in the container. Thereby, a temperature change within the insulating fluid representative to the whole container can be determined.
EP2 490 011 discloses a device that measures gas present above the insulation fluid in a transformer. The measurements are performed on top of the transformer.
CH212727 discloses a device for measuring pressure in insulation fluid in a transformer. The fluid is conducted in a pipe towards an extension vessel arranged above the transformer. A pressure sensor is present in the pipe that contains the fluid. The sensor is positioned above the housing of the transformer.
JP59021009 discloses a system for filtering particles from insulation oil of a transformer. The oil is conducted in a pipe outside the housing through a filter and conducted back into the housing. A pump on the pipe provides the power needed for conducting the fluid back into the housing. Pressure is measured using a sensor on top of the housing. JP56101718 discloses a system for cooling insulation oil of a transformer. The oil is conducted in a pipe outside the housing through a cooler and conducted back into the housing. A pump on the pipe provides the power needed for conducting the fluid back into the housing. The amount of particles in the fluid are measured before the fluid enters the housing.
JP56101718, discloses a system for cooling insulation oil of a transformer. The oil is conducted in a pipe outside the housing through a cooler and conducted back into the housing. A pump on the pipe provides the power needed for conducting the fluid back into the housing. The amount of particles in the fluid are measured before the fluid enters the housing.